use Map instead of Array with sort

 const input = 'i wanter Cats and dogs.'

const convert = (input)=>{

const arr = input.substring(0, input.length - 1).split(' ')

const result = new Map()

arr.forEach(item=> {

const key = item.length

if (result.has(key)){

result.set(item.length, result.get(key) + ' ' + item)

} else

result.set(item.length, item)

})

const sorted = new Map([...result.entries()].sort())

let sentense = ''

const values = Array.from(sorted.values())

values.forEach((value, i)=>{

const word = value.toLowerCase()

if (i ===0 )

sentense = sentense.concat(word.substring(0,1).toUpperCase()).concat(word.substring(1))

else

sentense = sentense.concat(word)

if (i === values.length -1)

sentense = sentense.concat('.')

else

sentense = sentense.concat(' ')

})

return sentense

}

console.log(convert(input))